This invention relates to a molding process for forming a polymeric layer on a substrate whereby the polymeric layer has a substantially uniform thickness within certain specified close tolerances. More specifically, this invention relates to a molding process, as aforesaid, wherein the polymeric layer is optically transparent. Even more specifically, this invention relates to a molding process for preparing a dust cover for an information disc.
Many molded articles have protective layers, which may be polymeric layers, as a coating on the molded article. In some instances, dimensional control of the thickness of the polymeric coating is not significant. In other instances, such as when the polymeric coating is to serve as a dust cover for an information disc, dimensional control of the thickness of the polymeric coating affixed to the information disc becomes important because these discs are read by a laser and dimensional variations in the thickness of the polymeric coating will interfere with the laser reading of the disc.
This invention, although applicable to applying a polymeric coating to any substrate for any purpose whatsoever, is particularly applicable to polymeric coatings for information storage devices which, may have a shape other than that of a disc. The invention will be described in connection with an information disc because the uniform thickness of the polymeric coating and the optical transparency, particularly laser light transparency, of said coating render the invention particularly suitable for use with information storage devices such as an information disc.
Coatings affixed to a substrate may be prepared by dipping the substrate into a coating composition, gluing a preformed coating onto the substrate, rolling a coating composition onto the substrate or by molding a coating composition onto a substrate itself. In some instances, a polymeric coating composition is dissolved in a solvent and the substrate is then coated with the solution and the solvent allowed to evaporate. In other instances, the coating composition, sans solvent, is placed on the substrate and the composition is then cured to form a polymeric layer affixed to the substrate.
One process for forming a polymeric layer upon a substrate involves the use of a two part mold wherein deformable gaskets are provided between the mold halves. These deformable gaskets, which are responsive to the application of external pressure (such as clamping the mold halves), allow the two parts of the mold to come toward one another when the molding process is conducted. The reason the mold halves are moved toward one another is to compensate for shrinkage which occurs when the coating composition is cured. If this external pressure is not applied in a uniform manner, the surface quality of the resultant polymeric coating will be poor. In addition, this process, suffers from poor dimensional control which results in wide, undesired thickness variations.
U.S. Pat. No. 4,188,433 is directed to a record carrier in disc form having a cover layer thereon. The U.S. Pat. No. 4,188,433 teaches that it is undesirable to coat a record carrier with a lacquer and then cure it because the outer portion of the lacquer cures before the inner portion of the lacquer cures and that the curing time is too long. The U.S. Pat. No. 4,188,433 teaches that, as a result of this uneven curing, it is difficult to get a smooth layer and that the layer is uneven and may result in the warping of the record carrier or in delamination of the lacquer layer.
U.S. Pat. No. 4,038,663 relates to information discs in general. The U.S. Pat. No. 4,038,663 teaches that a preformed plastic foil is spaced from a disc and is then pulled to the disc by vacuum. This process is not a molding process because the plastic foil has been preformed. The process is, instead, a process for attaching a preformed plastic foil to a disc.
U.S. Pat. No. 4,190,621 is directed to a process for manufacturing bifocal lenses. The U.S. Pat. No. 4,190,621 teaches that the lower part of a mold can be moved to compensate for shrinkage when molding a plastic bifocal lens.
None of the aforedescribed molding processes compensated for shrinkage, during curing of a coating composition, by the free movement of the substrate, within the mold itself.
An object of this invention therefore is to provide a process for forming a polymeric layer on a substrate.
Another object of this invention is to provide a molding process for forming an optically transparent polymeric layer on a substrate.
Yet another object of this invention is to provide a process for forming an optically transparent polymeric layer upon a substrate wherein the polymeric layer has a precise thickness.
Still another object of this invention is to provide a process for forming an optically transparent polymeric layer upon an information storage device such as a disc.
Other objects and advantages will become apparent from the following more complete description and claims.